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Why is Mercury so dark?

Mercury’s bleak, airless surface is similar to the moon’s, so scientists have long been puzzled why the planet reflects so much less light than our lunar satellite. On average, material blasted across Mercury’s surface by relatively recent impacts of comets, asteroids, and other small bodies reflects only two-thirds as much light as freshly excavated material on the moon, previous studies have shown. One of the prime explanations for this low reflectivity—an abundance of minerals including the element iron, which strongly absorb certain wavelengths of light falling upon them—doesn’t fit in this instance, researchers say. That’s because Mercury’s brightness at one particular wavelength suggests that there’s less than 3% iron in its surface rocks. Now, a team suggests the blame lies with another element entirely—carbon. Comets, which by some estimates are about 18% carbon by weight, are a major source of the element. But a much larger source may be a persistent pummeling by tiny carbon-rich meteorites, which stem from cometary dust among other sources and strike Mercury (shown in 2008 by a probe orbiting the planet) about 50 times more often than they do the moon, the researchers estimate today in Nature Geoscience. Besides ending up in the glassy materials forged by impacts, the carbon liberated from the micrometeorites when they vaporize on impact may take the form of amorphous carbon (something akin to soot), graphite, and nanodiamonds—all of which are stable on the largely airless planet, despite its hellish temperatures in sunlit areas. So it’s not difficult to imagine that a planet smudged with soot and pencil lead would appear darker than expected.